The present invention relates to a volume flow control system within an enclosed pressurized delivery system. More specifically, the invention relates to a means for automatically controlling the relative volume flow ratio of liquid or gaseous materials within an enclosed pressurized tubing system incorporating valves for flow control. The present system has particular applicability to the control of the fuel-to-air ratio mix in an industrial furnace in addition to process heaters and boilers.
In an industrial furnace, fuels such as natural gas or fuel oils are ignited in the presence of an oxidant, typically air, to create a high temperature flame. Such furnaces are used in the steel industry to melt aluminum, to fire refractory, to reheat steel, and in heat treating.
The efficiency of such furnaces depends largely on the proper control of the fuel-to-air ratio mix during combustion. Optimum combustion efficiency occurs when the fuel-to-air ratio is stoichiometric, i.e. when the mixture is at the precise proportion where all of the fuel is chemically reacted with all of the available oxygen, so as to produce the maximum amount of heat energy available from the reaction. If the fuel-to-air ratio is poorly controlled, e.g., if the furnace is operated with excessive air, the combustion will be very inefficient. The excess air does not contribute to the combustion process yet absorbs much of the flame's heat. Therefore, the flame loses much of its available heat to the excess air. Such inefficiency translates into considerable additional operating costs. Thus, it is desirable to control the fuel-to-air ratio so as to minimize excessive air during combustion.
In some prior furnace systems, control over the fuel-to-air ratio is achieved by mechanically measuring the pressure differentials across an orifice in the flow conduits of the air and the fuel. The measured pressure differentials are then mechanically transmitted to a hydraulically or pneumatically controlled valve actuator which opens or closes a fuel or air valve in order to correct for imbalances in the fuel-to-air ratio. Such mechanical controls work well on small systems, but as heat requirements of systems increase, requiring larger pipe diameters, the size and cost of the mechanical regulators increases sharply. The springs and diaphragms of such mechanical regulators become so large that their mass affects the operation of the regulator. The result is a large and expensive regulator which does not provide satisfactory control.
It is common in modern burner systems to pre-heat the air used in the burner. Such systems typically use recuperators which transfer the heat from the exhaust to the combustion air in order to efficiently maximize the use of the available heat. Such pre-heated air systems are particularly challenging for mechanical regulators. Volume flow control systems assume that the density of the materials being controlled and monitored are constant so that there is a direct relationship between volume and mass. In pre-heated air systems this assumption is not valid and temperature-related changes in air density must be taken into account.
Mechanical regulators can be used in pre-heated air systems if they are set to measure air flow prior to heating. However, recuperator systems tend to develop leaks which cause a portion of the heated air to be lost before reaching the burner. Some mechanical temperature compensation techniques employ passing a sample of heated air over a thermostatic metal strip that controls a specially tapered plug which in turn controls the amount of pressure bleed from the air measurement side of a mechanical regulator. Such elements are cumbersome and involve extra piping and more parts which tend to wear out and age under the high operating temperatures.
Mechanical control systems otherwise suffer from undesirable performance factors. It is sometimes desirable to operate the furnace under conditions of low heat input which require excess air. Mechanical control systems are not available which can be automatically programmed for excess air adjustments. Also, mechanical controls are prone to sticking and wearing. Therefore, such systems do not offer reliable maintenance of the desired fuel-to-air ratio. Thus, strictly mechanical control systems offer limited performance options and are unsatisfactory for modern burner systems.
In typical modern burner systems, the limitations of mechanical systems are avoided by using digital computer combustion control systems. Such systems use a distributed array of electronic sensors which monitor various parameters such as air temperature, air flow, fuel flow, and the percent of oxygen present in the exhaust gases. This sensor data is fed back to a computer microprocessor which calculates and implements the fuel or air flow adjustments needed to maintain the desired fuel-to-air ratio.
While digital computer systems offer greater control over combustion and all the related parameters, such systems are highly complex, requiring a network of sensors to be distributed over a large area. Also, the various components are bulky and expensive. Thus, computer combustion control, while offering benefits over the prior mechanical controls, also suffers from its own drawbacks.